1. Technical Field
The present disclosure relates to a terminal and a handover judgement method.
2. Description of the Related Art
Measurement Events
In LTE-Advanced system, which is an extension of 3GPP-LTE (3rd Generation Partnership Project Radio Access Network Long Term Evolution), a terminal (also referred to as user equipment (UE)) performs an operation described below. That is, when the terminal tries to change (make a handover) a base station (also referred to as a cell or eNB) to which the terminal is connected, the terminal measures reference signal received quality (RSRQ) of a cell-specific reference signal (CRS) transmitted from a base station that is a handover candidate cell, or reference signal received power (RSRP).
The terminal then judges an occurrence of one of measurement events including a threshold event and a comparison event as described below. For example, the terminal judges whether RSRQ/RSRP of a handover candidate cell has become greater than a threshold value (threshold event), or whether RSRQ/RSRP of the handover candidate cell has become greater than a value equal to RSRQ/RSRP of a cell to which the terminal is being connected (that is, a base station to which the terminal is being connected (also referred to as a serving base station (cell)) plus an offset value (comparison event). When a measurement event occurs, the terminal reports the occurrence of the measurement event (trigger of a handover) to the serving cell. When the base station receives the report of the occurrence of the measurement event, the base station performs a handover for the terminal (for example, see 3GPP TS36.331v11.6 (2013-09), “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control Protocol (Release 11)”, September 2013 (hereinafter referred to simply as NPL 1).
The type of the measurement event, parameters such as the threshold value used in the measurement event, the offset value, and the like, and a measured value (RSRP or RSRQ) used in the measurement event are set by the base station to the individual terminal.
Discovery Signal for Small Cell
In LTE-Advanced, it is under consideration to dispose a small cell which is a base station with a relatively low transmission power in a coverage area of a macrocell which is a base station with a relatively high transmission power thereby handling increasing traffic.
When a small cell is introduced, to suppress interference from the small cell and to reduce consumption power of the small cell, it is under consideration to control switching between an on-state and an off-state of the small cell (hereinafter referred to simply as the on/off control). In the off-state, the small cell is in a “hibernation state” in which no data is assigned to terminals. However, if transmission of all signals from the small cell is stopped, it becomes impossible for terminals to detect the small cell. To handle the above situation, it is under consideration to transmit a cell detection reference signal (hereinafter also referred to as a discovery signal) from the small cell thereby making it possible for terminals to detect the small cell even in the off-state.
In controlling the handover, the discovery signal is used, as with CRS, for measuring RSRQ/RSRP. However, to suppress the consumption power, it is under consideration to reduce the repletion frequency of transmission (to increase the transmission interval) of the discovery signal compared with CRS. Furthermore, as design of the discovery signal, it is under consideration to change the repletion period, the band, or the like of an existing signal used in the LTE-Advanced system thereby using it as the discovery signal. Candidates for a signal used as the discovery signal includes PRS (Positioning Reference Signal), CSI-RS (Channel-State Information Reference Signal), etc. (see, for example, 3GPP TS36.211v11.4.0 (2013-09), “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation (Release 11)”, September 2013, and 3GPP TSG RAN WG1 meeting, R1-133457, NTT DOCOMO, “Small Cell Discovery for Efficient Small Cell On/Off Operation”, August 2).
To suppress the consumption power, cells being in the off-state transmit only the discovery signal and transmits no other signal such that only terminals supporting RSRQ/RSRP measurement using the discovery signal (for example, terminals supporting the LTE standard Rel. 12) are allowed to detect cells in the off-state. On the other hand, cells in the on-state transmit the discovery signal and CRS, even terminals that do not support the RSRQ/RSRP measurement using the discovery signal (for example, terminals supporting the LTE standard Rel. 11 or older versions) are allowed to detect cells in the on-state.